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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,603 records · Page 89

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NASA NTRS1984

Modular approach for satellite communication ground terminals

The trend in satellite communications is toward completely digital, time division multiple access (TDMA) systems with uplink and downlink data rates dictated by the type of service offered. Trunking terminals will operate in the 550 MBPS (megabit per second) region uplink and downlink, whereas customer premise service (CPS) terminals will operate in the 25 to 10 MBPS region uplink and in the 200 MBPS region downlink. Additional criteria for the ground terminals will be to maintain clock sychronization with the system and burst time integrity to within a matter of nanoseconds, to process required order-fire information, to provide adaptive data scrambing, and to compensate for variations in the user input output data rates, and for changes in range in the satellite communications links resulting from satellite perturbations in orbit. To achieve the required adaptability of a ground terminal to the above mentioned variables, programmable building blocks can be developed that will meet all of these requirements. To maintain system synchronization, i.e., all bursted data arriving at the satellite within assigned TDMA windows, ground terminal transmit data rates and burst timing must be maintained within tight tolerances. With a programmable synchronizer as the heart of the terminal timing generation, variable data rates and burst timing tolerances are achievable. In essence, the unit inputs microprocessor generated timing words and outputs discrete timing pulses.

Gould, G. R.↗

NASA NTRS1984

Optical pulse generator using liquid crystal light valve

Numerical optical computing is discussed. A design for an optical pulse generator using a Hughes Liquid crystal light valve and intended for application as an optical clock in a numerical optical computer is considered. The pulse generator is similar in concept to the familiar electronic multivibrator, having a flip-flop and delay units.

Collins, S. A., Jr.↗

NASA NTRS1984

Doppler radar having phase modulation of both transmitted and reflected return signals

A microwave radar signal is generated for transmission through an antenna. Before transmission, the signal is phase modulated by 0 deg or 90 deg amounts during each alternate half-cycles of an intermediate frequency (IF) clock signal. After transmission and return, the signal is again phase modulated the same amounts during each alternate half-cycles. The return phase modulated signal is mixed with a leakage signal component of the microwave signal, leaving an IF Doppler. The IF Doppler signal may then be amplified, removing any requirement that direct current level signals be amplified and also removing the effect of detector noise from the Doppler signal.

Kobayashi, H. S.↗

NASA NTRS1983

Overview of Code Verification

The verified code for the SIFT Executive is not the code that executes on the SIFT system as delivered. The running versions of the SIFT Executive contain optimizations and special code relating to the messy interface to the hardware broadcast interface and to packing of data to conserve space in the store of the BDX930 processors. The running code was in fact developed prior to and without consideration of any mechanical verification. This was regarded as necessary experimentation with the SIFT hardware and special purpose Pascal compiler. The Pascal code sections cover: the selection of a schedule from the global executive broadcast, scheduling, dispatching, three way voting, and error reporting actions of the SIFT Executive. Not included in these sections of Pascal code are: the global executive, five way voting, clock synchronization, interactive consistency, low level broadcasting, and program loading, initialization, and schedule construction.

Source record↗

NASA NTRS1984

Time Interval Errors of a Flicker-noise Generator

Time interval error (TIE) is the error of a clock at time t after it is synchronized and syntonized at time zero. Previous simulations of Flicker FM noise yielded a mean-square TIE proportional to sq t. It is shown that the order of growth is actually sq t log t. The earlier sq t result is explained and a modified version of the Barnes-Jarvis simulation algorithm is given.

Greenhall, C. A.↗

NASA NTRS1984

Integrating IR detector imaging systems

An integrating IR detector array for imaging is provided in a hybrid circuit with InSb mesa diodes in a linear array, a single J-FET preamplifier for readout, and a silicon integrated circuit multiplexer. Thin film conductors in a fan out pattern deposited on an Al2O3 substrate connect the diodes to the multiplexer, and thick film conductors also connect the reset switch and preamplifier to the multiplexer. Two phase clock pulses are applied with a logic return signal to the multiplexer through triax comprised of three thin film conductors deposited between layers. A lens focuses a scanned image onto the diode array for horizontal read out while a scanning mirror provides vertical scan.

Bailey, G. C.↗

NASA NTRS1984

Timing performance of phased-locked loops in optical pulse position modulation communication systems

An optical digital communication system requires that an accurate clock signal be available at the receiver for proper synchronization with the transmitted signal. Phase synchronization is especially critical in M-ary pulse position modulation (PPM) systems where the optimum decision scheme is an energy detector which compares the energy in each of M time slots to decide which of M possible words was sent. Timing errors cause energy spillover into adjacent time slots (a form of intersymbol interference) so that only a portion of the signal energy may be attributed to the correct time slot. This effect decreases the effective signal, increases the effective noise, and increases the probability of error. A timing subsystem for a satellite-to-satellite optical PPM communication link is simulated. The receiver employs direct photodetection, preprocessing of the detected signal, and a phase-locked loop for timing synchronization. The variance of the relative phase error is examined under varying signal strength conditions as an indication of loop performance, and simulation results are compared to theoretical calculations.

Lafaw, D. A.↗

NASA NTRS1983

SIFT - A preliminary evaluation

This paper presents the results of a performance evaluation of the SIFT computer system conducted in the NASA AIRLAB facility. The essential system functions are described and compared to both earlier design proposals and subsequent design improvements. The functions supporting fault tolerance are found to consume significant computing resources. With SIFT's specimen task load, scheduled at a 30-Hz rate, the executive tasks such as reconfiguration, clock synchronization and interactive consistency, require 55 percent of the available task slots. Other system overhead (e.g., voting and scheduling) use an average of 50 percent of each remaining task slot.

Palumbo, D. L.↗

NASA NTRS1982

Orbit determination of low-altitude earth satellites using GPS RF Doppler

This paper investigates the use of one type of GPS-signal derived measurement, the doubly differenced GPS RF Doppler, in the orbit determination of low-altitude earth satellites. This measurement is simple to make, is insensitive to clock errors and provides a continuous determination of the user satellite orbit. Analysis shows that, with a constellation of 18 GPS satellites and 13 ground stations, the 1300 km altitude of a user satellite (TOPEX) can be determined to 5 cm in 2 hours of tracking. Examination is made of the effects of employing fewer ground stations, of different solution strategies, and of introducing fictitious thrust parameters to alleviate the geopotential modeling error which is one of the major error sources.

Wu, S. C.↗

NASA NTRS1982

Serial MSK modem for the Advanced Communications Satellite Program

The design and test results of the 550-Mb/s, 110-Mb/s, and 27.5-Mb/s modems for the Advanced Communication Satellite Program are described. The TDMA/FDMA satellite environmental constraints have resulted in several unique features which include total acquisition times of 100 bit times and modem degradation of less than 1.5 dB at 10 to the -6th Pe. Key to the serial implementation of MSK is the resulting hardware simplicity and degradation insensitivity relative to conventionally implemented MSK and QPSK modems. The 110/27.5-Mb/s modems use an integrated circuit specifically developed for the program to perform the key carrier and clock-loop functions.

Stilwell, J. H.↗

NASA NTRS1984

Hy-wire measurements of atmospheric potential

A method for directly measuring the electric potential drop across the lowest portion of the atmosphere by using an apparatus called Hy-wire is described. The Hy-wire system consists of a long, high-voltage, insulated conductor connected at the top to a wire-mesh braid and terminated at the bottom inside a large-diameter corona ball mounted on 1 m Teflon legs. The system impedance is near 10 billion ohms, while the grounded mode, Hy-wire draws about 10 micro-a at 500 m altitude. Around-the-clock measurements with characteristic peaks near 1530 and 1900 UT of potentials near 125,000 volts are presented, with total daily average potential near 90,000 volts at 507 m altitude. The experimental weather conditions varied from clear to local fog, except that there was only trace precipitation. The data have been interpreted in terms of the possible source of the potential variations; neither a local nor a global source is clearly indicated.

Holzworth, R. H.↗

NASA NTRS1984

ACTS TDMA network control

This paper presents basic network control concepts for the Advanced Communications Technology Satellite (ACTS) System. Two experimental systems, called the low-burst-rate and high-burst-rate systems, along with ACTS ground system features, are described. The network control issues addressed include frame structures, acquisition and synchronization procedures, coordinated station burst-time plan and satellite-time plan changes, on-board clock control based on ground drift measurements, rain fade control by means of adaptive forward-error-correction (FEC) coding and transmit power augmentation, and reassignment of channel capacities on demand. The NASA ground system, which includes a primary station, diversity station, and master control station, is also described.

Inukai, T.↗

NASA NTRS1983

Redundant asynchronous microprocessor system for fault tolerant flight control and navigation

Unlike their synchronized counterparts, redundant channels in an asynchronous flight system can, under no-fault conditions, exhibit cross-channel data disparities. Sources of these errors are examined in terms of the general, individual functions of the flight control and navigation application in the asynchronous digital environment. The effects of asynchronism on trajectory programmers, dynamic control algorithms and data reconstruction processes are examined in terms of data skews, data latencies, and clock rate uncertainties. An example is presented in which time corrections are applied to reduce the data disparities. Practical limitations of the approach of the example are discussed.

Dunn, W. R.↗

NASA NTRS1984

Performance of a dedicated VLBI system for TDRSS navigation

Results of a covariance study to evaluate the performance of a proposed Very Long Baseline Interferometric (VLBI) system for TDRSS orbit determination (OD) are presented. This dedicated VLBI system consists of 4 one meter antennas for each TDRS positioned on orthogonal 6000 km baselines and uses a GPS signal for clock synchronization and calibration of measurement error sources. The performance of the VLBI system is compared with the current BRT system for support of TDRSS navigation. The criteria for evaluating the system performance is the TDRSS OD accuracy and the resulting navigation accuracy for two typical TDRSS users represented by a TOPEX type orbit and a Space Shuttle orbit. Based on the anticipated characteristics of a 1990 era VLBI system a factor of 20 improvement can be expected in the TDRSS OD accuracy. This translates into accuracy improvements ranging from factors of 3 to 10 for typical TDRSS users.

Ellis, J.↗

NASA NTRS1985

Synchronization for Optical PPM Signals

Method based on statistical properties of weak pulse-positionmodulated (PPM) signal enables synchronization of receiver clock with received-signal time base. Method applies to weak optical M-ary PPM signals, for which there is only one pulse of length Tp transmitted during one of timeslots of length T in each successive interval of M timeslots. Method requires small dead time, Td, at beginning and end of each timeslot, during which pulse amplitude is zero.

Vilnrotter, V. A.↗

NASA NTRS1985

Doubly-Differenced Measurements for Orbit Determinations

Report describes accurate method of determining orbit of low-altitude Earth satellite. Method insensitive to clock errors and provides continuous determination of orbit. Report examines effects of employing fewer ground stations, applying different solution strategies, and introducing fictitious thrust parameters to minimize geopotential modeling error - one of major error sources.

Wu, S. C.↗

NASA NTRS1985

Fast Control Sequencer

Unit faster than conventional circuits and contains fewer integrated circuits. Fast logic controller accommodates three delays before second half cycle of clock signal strobes status latch: Outputs of pipeline register must settle; selected input status line signal must propagate through status multiplexer and arrive at status latch; and it takes 3 nanoseconds for status latch set up before strobed. During second half cycle, output-enable line produces next instruction and applies it to pipeline register.

Lahmeyer, C. R.↗

NASA NTRS1984

Status of the National Space Transportation System

The National Space Transportation System is a national resources serving the government, Department of Defense and commercial needs of the USA and others. Four orbital flight tests were completed July 4, 1982, and the first Operational Flight (STS-5) which placed two commercial communications into orbit was conducted November 11, 1982. February 1983 marked the first flight of the newest orbiter, Challenger. Planned firsts in 1983 include: use of higher performance main engines and solid rocket boosters, around-the-clock crew operations, a night landing, extra-vehicular activity, a dedicated DOD mission, and the first flight of a woman crew member. By the end of 1983, five commercial payloads and two tracking and data relay satellites should be deployed and thirty-seven crew members should have made flights aboard the space shuttle.

Abrahamson, J. A.↗